Sharks play a pivotal role as keystone species in marine ecosystems, yet many shark populations are now facing severe declines, underscoring the importance of advancing our understanding of their health and immune biology. Beyond conservation, sharks hold unique evolutionary significance, as they represent one of the earliest vertebrate lineages to possess a fully developed immune system encompassing both innate and adaptive arms, including MHC class I and II molecules and recombination-activating genes (RAG). Despite this, a high-resolution reference of shark immune cell composition has been lacking. Here, we present a single-nucleus RNA sequencing (snRNA-seq) analysis of peripheral blood mononuclear cells (PBMCs) from the cloudy catshark (Scyliorhinus torazame) following antigenic stimulation with SARS-CoV-2 receptor-binding domain (RBD). Unsupervised clustering revealed multiple distinct immune cell populations, enabling the identification of conserved vertebrate immune lineages as well as shark-specific immune cell subsets. Transcriptomic profiling further uncovered lineage-specific gene expression programs that highlight both shared and unique features of shark immunity. This study provides the first single-cell–level immune atlas of sharks, offering novel insights into shark immune biology, disease susceptibility, and evolutionary immune system design. Our findings establish a foundational resource for future comparative, veterinary, and translational immunology studies, with implications spanning wildlife conservation and evolutionary medicine.